Our objectives are: 1) detailed studies of the structure, interactions, functions, and biosynthesis of membranes; and 2) the development and application of new and improved methods for ultrastructure studies. In the first category, studies will be carried out: a) on the molecular mechanisms of ligand-induced endocytosis in fibroblasts and other cells; b) on the role of a protein named vinculin, recently discovered in our laboratory, in the attachment of microfilaments to membranes; and c) on the intracellualr pathways of membrane integral proteins from their sites of synthesis to the plasmalemma. For all of these studies, we will employ, and continue to improve, methods developed in our laboratory for the immunoelectron microscopic labeling of specific proteins inside cells and tisues prepared by ultrathin frozen sectioning.